Implement upstream RequestDevice, add native-only CreateDevice

This CL implements RequestDevice and also has changes for
Dawn to internally use wgpu::FeatureName enums, instead of
strings. Some of the string handling is kept for now to
support the deprecated creation path. GetFeatureInfo is added
to the instance to get a name and description of the feature,
for reporting in about://gpu.

Dawn device toggles are now passed in an extension struct off
of the device descriptor. This is only supported in dawn_native,
and not dawn_wire, for now, since dawn_wire doesn't have a way
to serialize lists of null-terminated const char*.

To enable the client to check whether the toggle descriptor is
supported, a `dawn-native` feature is added which is supported
all the time with dawn_native, but not supported with dawn_wire.

Feature `dawn-native` also enables a synchronous version of
CreateDevice for convenience.

Bug: dawn:160
Change-Id: Ifc195e7ea808c6c319021528ef4b36bd65583bff
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/72020
Reviewed-by: Loko Kung <lokokung@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>

src/include/dawn_native/DawnNative.h

@@ -121,15 +121,17 @@ namespace dawn_native {
 
         explicit operator bool() const;
 
-        // Create a device on this adapter, note that the interface will change to include at least
-        // a device descriptor and a pointer to backend specific options.
-        // On an error, nullptr is returned.
-        WGPUDevice CreateDevice(const DawnDeviceDescriptor* deviceDescriptor = nullptr);
+        // Create a device on this adapter. On an error, nullptr is returned.
+        WGPUDevice CreateDevice(const DawnDeviceDescriptor* deviceDescriptor);
+        WGPUDevice CreateDevice(const WGPUDeviceDescriptor* deviceDescriptor = nullptr);
 
         void RequestDevice(const DawnDeviceDescriptor* descriptor,
                            WGPURequestDeviceCallback callback,
                            void* userdata);
 
+        // Returns the underlying WGPUAdapter object.
+        WGPUAdapter Get() const;
+
         // Reset the backend device object for testing purposes.
         void ResetInternalDeviceForTesting();
 
@@ -173,6 +175,7 @@ namespace dawn_native {
         std::vector<Adapter> GetAdapters() const;
 
         const ToggleInfo* GetToggleInfo(const char* toggleName);
+        const FeatureInfo* GetFeatureInfo(WGPUFeatureName feature);
 
         // Enables backend validation layers
         void EnableBackendValidation(bool enableBackendValidation);